Ultrasonic vibration mode for wire bonding

ABSTRACT

A method and apparatus for creating second order vibrational modes. The apparatus includes a signal generator, a piezoelectric transducer, a plurality of wave propagating beams and reflecting boards. An electric field applied by the signal generator to the piezoelectric transducer induces a unidirectional vibration of the transducer. The vibration is propagated through the beams and reflected by the reflecting boards in a closed polygonal loop. The final reflection direction is perpendicular to the original vibration. A circular or elliptical vibration of the apparatus results. The circular or elliptical vibrational energy can be imparted to the wire bond of an integrated circuit to add strength to the connection.

This is a continuation of application Ser. No. 09/293,676 filed Apr. 16,1999 now U.S. Pat. No. 6,244,498.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of semiconductor packagewire bonding, and, more particularly, to a novel apparatus and methodfor producing an ultrasonic vibration mode to improve the bond between awire and a die or lead.

2. Description of the Related Art

Wire bonding today is used throughout the microelectronics industry as ameans of interconnecting chips, substrates, and output pins. Automaticultrasonic gold ball bonding is a high yield interconnect process thatuses heat and ultrasonic energy to form a metallurgical bond. Typically,high purity gold wire is used with a ball bond formed at one end and astitch bond at the other. FIGS. 1a through 1 g show the typical sequenceof steps involved in forming a gold ball bond. FIG. 1a shows a capillary10 which is targeted on the bond pad and positioned above a die 12 witha ball 14 formed on the distal end of a wire 16 and pressed against theface of capillary 10. Capillary 10 descends as shown in FIG. 1b,bringing ball 14 into contact with die 12. The inside radius ofcapillary 10 grips ball 14 in forming the bond. Ultrasonic vibrationenergy is then applied. The ultrasonic vibration energy is typicallyproduced by piezoelectric transducers. Piezoelectric transducers arewell known in the industry and comprise a piezoelectric material, i.e. amaterial that converts mechanical energy into electrical energy and viceversa. In the case of producing ultrasonic vibration energy, an electricfield is applied to a piezoelectric ceramic to stimulate vibration.After ball 14 is bonded to die 12 with the aid of the ultrasonicvibration energy, capillary 10 raises to the loop height position asshown in FIG. 1c. A clamp 18 is then opened and wire 16 is free to feedout of the end of capillary 10. Next, a lead 20 of the device ispositioned under capillary 10 and capillary 10 is lowered to the lead.Wire 16 is fed out the end of capillary 10, forming a loop as shown inFIG. 1d. The capillary continues downward and deforms wire 16 againstlead 20, producing a wedge-shape bond which has a gradual transitioninto the wire as shown in FIG. 1e. Ultrasonic vibration energy is onceagain applied to enhance the bond strength. Capillary 10 then raises offlead 20 as shown in FIG. 1f, leaving a stitch bond. At a pre-set height,clamp 18 is closed while capillary 10 is still rising with the bondinglead. This prevents wire 16 from feeding out capillary 10 and producesan upward force on the bond. The force builds until wire 16 breaks,which it does at the smallest cross section of the bond. Finally, a newball 14 is formed on the new distal end of wire 16 by employing ahydrogen flame or an electronic spark as shown in FIG. 1g. The processcan then be repeated.

Ultrasonic aluminum wire bonding is also a widely used high speed, highthroughput interconnect process. In this process, stitch bonds such asdescribed above with reference to FIG. 1f are formed at both ends of theinterconnect by a combination of pressure and ultrasonic energy. As thewire softens, freshly exposed metal in the wire comes in contact withthe freshly exposed metal on the pad and a metallurgical bond is formed.Aluminum wire is typically doped with silicon (e.g., 1%) to more closelymatch the hardness of the wire with the bond pad material. Both gold andaluminum wire are used extensively today in packaging, with gold ball toaluminum bond pads being the most common interconnect system.

In conventional wire bonding processes, it is well known by those in theart that bonding strength is enhanced by employing ultrasonic vibrationand heat during the bonding procedure and this is typically done. Thestrength of the bond is only enhanced, however, in the same direction asthe ultrasonic vibration being applied. Current processes typicallyapply only unidirectional vibrations during wire bonding, whereas itwould be desirable to enhance the bond strength in all directions. Inaddition, in order to ensure that integrated circuits are not degradedduring the attachment of the bonding wires, it is desirable to conductthe ultrasonic wire bonding at relatively low temperatures. However, thelower the temperature, the more difficult it may be to form a sufficientbond. Therefore there is a continuing need to create better bonds atlower temperatures and at faster rates to increase productivity.

It has been proposed that the application of ultrasonic waves that arecircular or elliptical can enhance the bond strength at lowertemperatures and with a shorter dwell time, in each of the vibrationdirections. See e.g., Tsujino, “Ultrasonic wire bonding using highfrequency 330, 600 kHz and complex vibration 190 kHz welding systems”(Ultrasonics 34 (1996) 223-228). This strengthening phenomenon haspurportedly been achieved by producing the circular or ellipticalvibration modes using multiple piezoelectric transducers. It isparticularly desirable to generate a circular or elliptical vibrationmode for better bond strength in all directions. However, current knownmethods for producing complex ultrasonic waves using multipletransducers typically employ separate, non-synchronous controls for eachtransducer, such that error or other difficulties may be introduced bythe two separate controls that do not work together and result in a lessthan ideal higher order wave. In addition, a single transducer with asingle control apparatus would be less expensive than two transducerswith separate controls. There is a need for production of circular orelliptical ultrasonic vibrations with a mechanism that ensures that thetwo perpendicular modes needed for circular or elliptical modes arealways vibrating synchronously.

SUMMARY OF INVENTION

In accordance with one aspect of the present invention, a novelultrasonic vibration mode for wire bonding is provided. A secondvibration direction is added to the conventional ultrasonic apparatus.The combined elliptical or circular vibration enhances wire bondstrength in all directions and decreases bonding time and temperature.The second vibration direction is created with a single piezoelectrictransducer and control mechanism in combination with a reflecting armapparatus. The present invention enables two perpendicular wave modes toalways vibrate synchronously. The invention can advantageously beapplied to any ultrasonic bonding process to improve yield strength andreliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the invention willbecome further apparent upon reading the following detailed descriptionand upon reference to the drawings in which:

FIG. 1a illustrates a conventional wire bonding apparatus with acapillary targeted on the bond pad and positioned above a die with aball formed on the end of a wire.

FIG. 1b illustrates the conventional wire bonding apparatus from FIG. 1aafter the capillary has descended, bringing the ball into contact withthe die.

FIG. 1c illustrates the conventional wire bonding apparatus from FIG. 1aafter rising to the loop height position.

FIG. 1d illustrates the conventional wire bonding apparatus from FIG. 1arepositioned over the lead.

FIG. 1e illustrates the conventional wire bonding apparatus with thecapillary deforming the wire against the lead.

FIG. 1f illustrates the conventional wire bonding apparatus from FIG. 1awith the capillary raised off the lead and leaving a stitch bond.

FIG. 1g illustrates the conventional wire bonding apparatus from FIG. 1awith a new ball formed on the tail of the wire.

FIG. 2 is a top view of a piezoelectric transducer with a triangularshaped arm attached, in accordance with one embodiment of the invention.

FIG. 3 is a side view of the piezoelectric transducer and attached arm,from FIG. 2.

FIG. 4 illustrates a circular wave mode produced by the transducer/armapparatus from FIG. 2.

FIG. 5 illustrates an elliptical wave mode produced by thetransducer/arm apparatus from FIG. 2.

FIG. 6 is a top view of an alternative embodiment of the piezoelectrictransducer and attached arm.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, that will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Turning now to the drawings, and in particular to FIG. 2, one embodimentof the air invention is disclosed. In the embodiment of FIG. 2, apiezoelectric transducer 22 is mechanically attached to a polygonal arm24. Polygonal arm 24, for example the substantially triangular-shapedarm 24 depicted in FIG. 2, comprises a medium through which theultrasonic waves are capable of traveling when such waves are initiatedby transducer 22. Arm 24 may comprise stainless steel or some othercompatible material. Arm 24 includes multiple beams defined by closedconnections, for example three beams 28, 30 and 34 and three connectionpoints 26, 32 and 36. In an alternative embodiment shown in FIG. 6, arm24 comprises a single beam formed into a polygonal shape. The individuallengths and number of beams may be arbitrary, but in accordance with thepresently disclosed embodiment of the invention, the total length of thebeams should yield:

TotalLength=(n±¼)λ

Where λ is the ultrasonic wavelength in the beam media and n=0, 1, 2 . ..

In the embodiment of FIG. 2, point 26 joins beam 28 with beam 30, point32 joins beam 28 with beam 34, and point 36 joins beam 34 with beam 30.At point 32 and 36, fixed reflection boards 38 and 40, respectively,attach to arm 24 by a weld or some other attaching mechanism. Reflectionboards 38 and 40 will preferably comprise a material with an overallYoung's Modulus that is substantially different from the beam materialto promote optimum reflection of the wave from the original propagationdirection. In theory, reflection boards 38 and 40 are fixed such thatthey act as infinite walls and reflect 100% of the wave energy. Inpractice, however, reflection boards 38 and 40 may be connected to thewire bonding apparatus and absorb a portion of the vibrational energy.In one embodiment, the fixed reflection boards 38 and 40 each reflect anultrasonic wave 45° from the original wave propagation direction. In analternative embodiment, multiple reflection boards attached to anynumber of beams each reflect and redirect an ultrasonic wave someangular amount. In the alternative embodiment shown in FIG. 6, a singlepiece arm 24 is only fixed to transducer 22, with no reflection boards.In the alternative embodiment of FIG. 6, the vibrational energy ispropagated along the path of a single beam. In one embodiment theultrasonic wave is preferably reflected or re-directed approximately 90°or some odd multiple of 90°.

Point 26 also describes the location of capillary 10, which is normal tothe page in FIG. 2. Capillary 10 can be clearly seen in FIG. 3 extendingdown from point 26. As discussed above, capillary 10 houses wire 16. Inthe embodiment shown in FIG. 3, capillary 10 is orthogonal to the planedescribed by polygonal arm 24. FIG. 3 illustrates piezoelectrictransducer 22 and arm 24 from a side view with beam 28 of arm 24extending longitudinally from the end of piezoelectric transducer 22.Beam 28 terminates at fixed reflection board 38. FIG. 3 also shows anorthogonal view of the circular trajectory 42 that point 26 andcapillary 10 follow as vibration in perpendicular directions along x andy axes 29 and 31 occurs.

Piezoelectric transducer 22 produces ultrasonic waves in response to anelectrical field applied to the transducer by a signal generator 23.Signal generator 23, for example the standard signal generator that isincluded with a K & S 8000 Series wire bonding machine, can apply drivesignals to the transducer. However, a single transducer can only produceunidirectional vibration waves, i.e. vibration in one planar direction,without the aid of arm 24. Operation of the device to produce a circularor elliptical ultrasonic vibration mode, i.e. vibration in twoperpendicular planes, is as follows: An electric field is applied totransducer 22, which induces a unidirectional, longitudinal vibration ofthe piezoelectric material. Those of ordinary skill in the art willappreciate that the vibration is an ultrasonic wave that corresponds infrequency and amplitude to the electric field it is subjected to. Themost common frequency is 60 kHz, but higher frequencies such as 120 kHzare becoming more common. The unidirectional, longitudinal vibration iscommunicated from the end of transducer 22 at point 26 to arm 24. Point26 is thus vibrating at a certain frequency and amplitude longitudinallyalong beam 28 according to the vibration direction produced by thetransducer. The axis defined by beam 28 is referred to herein as thex-axis 29. The wave travels through beam 28 of arm 24 until it reachesfixed reflection board 38. The wave is reflected 45° at reflection board38 and continues along beam 34 until it reaches reflection board 40,where it is reflected another 45°. The wave continues along beam 30 ofarm 24 where it meets perpendicularly with the wave being generated bytransducer 22 at point 26. The axis defined by beam 30 is referred toherein as the y-axis 31. At point 26 there is now a circular orelliptical wave mode produced with the characteristics exhibited inFIGS. 4 and 5. The circular and elliptical paths shown in FIGS. 4 and 5represent the trajectories 42 and 44 of point 26 and capillary 10. Point26 and thus capillary 10 are vibrating in two perpendicular directions(the x and y directions) to produce the circular mode. In theory, thereflectors reflect 100% of the vibrational energy and the resultantvibration mode at point 26 is perfectly circular as shown in FIG. 4. Inreality, there may some energy lost to absorption by the reflectors andthe resulting vibration mode may be elliptical as shown in FIG. 5. Inthe alternative embodiment shown in FIG. 6, arm 24 comprises a singlebeam formed into a polygonal shape with no reflecting boards. In thisalternative embodiment the wave vibration is re-directed approximately90° along the beam path until it meets perpendicularly with the wavegenerated by transducer 22 at point 26 to create a circular orelliptical wave mode.

Those of ordinary skill in the art will appreciate that a circular orelliptical mode of all ultrasonic energy such as is provided by systemsin accordance with the presently disclosed embodiment of the inventionallows a wire being bonded to attach more securely in a shorter amountof time and/or at a lower temperature than conventional wire bonding.One potential advantage of a lower bonding temperature is the avoidanceof copper leadframe oxidation. Another potential advantage of a lowertemperature is the fact that at high temperatures, polymeric dieadhesive absorbs ultrasonic energy, and it is desirable minimize thisabsorption so that more ultrasonic energy is absorbed by the bond, thusincreasing bond strength. One reason the circular or elliptical modeallows wire bonding to be done in a shorter time is that, as notedabove, bond strength is only increased in the direction of theultrasonic vibration. If the vibration is unidirectional, the bond isonly strengthened in one direction; hence, ultrasonic energy must beapplied long enough to ensure the added bond strength in the onedirection is sufficient to compensate for the relatively weaker bondstrengths in other directions. With the circular or ellipticalvibrational mode created by systems in accordance with the presentlydisclosed embodiment of the invention, the bond is strengthened in alldirections, allowing shortened dwell time and reduced bondingtemperature.

FIG. 4 illustrates the theoretical vibration mode accomplished bysystems in accordance with the presently disclosed embodiment of theinvention. The circular mode is the general trajectory path 42 of point26. Point 26 exhibits movement in all directions of the x-y plane,strengthening the bond equally in each of the planar directions. It islikely, however, that there will be some energy lost to reflectionboards 38 and 40, or, in the alternative embodiment, to the single beam,and the resultant vibrational mode will be elliptical as shown in FIG.5. The elliptical trajectory 44 of point 26 also encompasses all the x-yplanar directions, however, the bond in x-direction 29 will be strongerthan the bond strength in y-direction 31 if the vibrational moderesembles elliptical shape of FIG. 5. This result occurs because of thegreater vibrational energy that is imparted in x-direction 29 than iny-direction 31 in the elliptical mode.

It will be understood that fixed arm 22 rigidly attached at a 90° angleto transducer 22 enables the vibration at point 26 to be a combinationof perpendicular waves, creating a circular or elliptical vibration modeand eliminating the need for precise control of dual transducers toensure the same result.

A method of creating the circular or elliptical ultrasonic vibrationmodes according to the present invention for use with wire bonding mayinclude: a) attaching one or more of beams in a polygonal armconfiguration, for example attaching beams 28, 34, and 38 in thesubstantially triangular configuration shown in FIG. 2; b) attaching thearm to a transducer, for example attaching arm 24 to transducer 22; c)reflecting vibrational energy generated by the transducer from one beamto the next with a reflecting board, for example reflecting thevibration in beam 28 to beam 34 via reflecting board 32; and d)arranging the beams such that the angle between the two beams at theattachment point of the arm to the transducer is 90° or some oddmultiple thereof, for example the 90° angle between beam 28 and beam 38at attachment point 26. The ultrasonic vibrational energy generated isimparted to the bonding site of a wire bonding apparatus.

A method for creating circular or elliptical ultrasonic vibration modesin an alternative embodiment may include: a) attaching a single beam ina closed arm configuration to a transducer, for example triangular arm46 attached to transducer 22 as shown in FIG. 6; b) redirecting aportion of the vibrational energy generated by the transducer along theclosed path defined by the arm, for example redirecting a portion of thevibrational energy along the triangular path defined by arm 46; and c)arranging the single beam arm such that the angle between the two distalends of beam at the arm/transducer attachment point is 90° or some oddmultiple thereof, for example the 90° angle shown at point 26 of FIG. 6between arm 46 and transducer 22.

A method for forming a bond between a wire and a conductive element at abond site incorporating the present invention may include: a) generatingultrasonic energy, for example the ultrasonic energy generated bypiezoelectric transducer 22; b) applying a first portion of theultrasonic energy directly to the bond site in a first direction, forexample applying the ultrasonic energy in x-direction 29 as shown inFIG. 3; c) redirecting a second portion of the ultrasonic energy along amedium capable of transmitting said ultrasonic energy, for exampleredirecting the vibrational energy along arm 24 as shown in FIG. 2; andd) applying the second portion of the ultrasonic energy to the bond sitein a second direction different from the first direction, for exampleapply the ultrasonic energy in y-direction 31 as shown in FIG. 3.

While the present invention has been particularly shown and describedwith reference to a particular illustrative embodiment thereof, it willbe understood by those skilled in the art that various changes in formand details may be made without departing from the spirit and scope ofthe invention. The above-described embodiment is intended to be merelyillustrative, and should not be considered as limiting the scope of thepresent invention.

What is claimed is:
 1. A wire bonding apparatus, comprising: a capillaryhaving a lumen therein adapted to receive a bonding wire therein, saidbonding wire having a ball formed on a distal end thereof extending atleast partially out of said capillary; an ultrasonic transducer, coupledto said capillary and adapted to impart ultrasonic vibrational energy tosaid capillary; wherein said ultrasonic transducer comprises: atransducer for generating vibrational energy; a signal generator forapplying drive signals to said transducer; a polygonal ultrasonicwave-propagating arm coupled to said transducer, said polygonal armcomprising a plurality of beams and at least one fixed reflection board,said beams and said at least one fixed reflection board cooperating toangularly reflect said vibrational energy such that said capillaryvibrates in a substantially circular or elliptical vibration mode.
 2. Awire bonding apparatus, comprising: a capillary having a lumen thereinadapted to receive a bonding wire therein, said bonding wire having aball formed on a distal end thereof extending at least partially out ofsaid capillary; an ultrasonic transducer, coupled to said capillary andadapted to impart ultrasonic vibrational energy to said capillary;wherein said ultrasonic transducer comprises: a transducer forgenerating vibrational energy; a signal generator for applying drivesignals to said transducer; an ultrasonic wave-propagating arm coupledto said transducer, said arm comprising a single beam configured in aclosed loop to angularly redirect said vibrational energy such that saidcapillary vibrates in a substantially circular or elliptical vibrationmode.
 3. A wire bonding apparatus in accordance with claim 1 or 2,wherein said transducer comprises a piezoelectric ceramic.
 4. A wirebonding apparatus in accordance with claim 1 or 2, wherein saidcapillary is adapted to target a bond pad on a silicon substrate.
 5. Awire bonding apparatus in accordance with claim 1 or 2, wherein said armhas a substantially triangular shape.
 6. A wire bonding apparatus inaccordance with claim 1 or 2, wherein said arm is mechanically coupledto said transducer.
 7. A wire bonding apparatus in accordance with claim6, wherein said capillary is mechanically coupled to said arm.
 8. A wirebonding apparatus in accordance with claim 7, wherein said capillaryvibrates in a substantially circular mode.
 9. A wire bonding apparatusin accordance with claim 7, wherein said capillary vibrates in asubstantially elliptical mode.
 10. A wire bonding apparatus inaccordance with claim 1, wherein the collective length of said beamscomprising said polygonal arm is governed by: TotalLength=(n±¼)λ where λis the ultrasonic wavelength in the beam media and n=0, 1, 2, . . . 11.A wire bonding apparatus in accordance with claim 1, wherein the totallength of said beam is governed by: TotalLength=(n±¼)λ where λ is theultrasonic wavelength in the beam media and n=0, 1, 2, . . .
 12. A wirebonding apparatus in accordance with claim 1, wherein said at least onereflection board is rigidly coupled to at least one of said plurality ofbeams.
 13. A wire bonding apparatus in accordance with claim 12, whereinsaid at least one reflection board is rigidly coupled to at least one ofsaid plurality of beams at a connection point between two of saidplurality of beams.
 14. A wire bonding apparatus in accordance withclaim 1, wherein said at least one reflection board comprises a materialwith an overall high Young's Modulus than the material comprising saidplurality of beams.